1. Field of the Invention
The present invention relates to filters for use in projection photolithographic system, and more particularly to a filter for use in a photolithographic system used in manufacturing a semiconductor device, or a photolithographic system which uses either a visible light or a ultraviolet light to project or transcribe a pattern on a mask for the photolithographic process.
2. Description of the Prior Art
On silicon substrates of very large scale integration (LSI) semiconductor devices manufactured in recent years, a plurality of transistors and wirings on the submicron order are integrated. For forming these fine patterns is used the photolithographic technique in which mask patterns are demagnified (usually to the size of one fifth of the original) and transcribed on a photosensitive (resist) resin films coating semiconductor substrates to form patterns on the submicron order.
Semiconductor memory devices such as 1M DRAM's and 4M DRAM's currently manufactured in large quantity use a minimum line width of 1.2 .mu.m and 0.8 .mu.m. Most of photolithographic systems used for manufacturing such semiconductor devices use luminous rays such as g-line having a wavelength of 436 nm and emitted from ultra-high pressure Hg lamps. Some of the photolithographic systems have just begun to use luminous rays such as i-line having a wavelength of 365 nm.
Semiconductor memory devices such as 16M DRAM's and 64M DRAM's to be manufactured in the future are expected to have a minimum line width of 0.6 to 0.5 .mu.m or 0.4 to 0.3 .mu.m. Manufacturing such semiconductor devices in large quantity requires the development of photolithographic technique having a higher resolution. For improving the resolution by shortening the wavelength of photolithographic light, use of i-line in the place of g-line, and even use of Krypton-fluorine (KrF) excimer laser are currently studied.
These new photolithographic systems illuminate the rear surface of masks on which are arranged opaque patterns normally formed of sheet metal on transparent substrates like quartz and demagnify a luminous flux passing through the masks thereby forming an image on semiconductor substrates.
FIGS. 12a and 12b shows one example schematically illustrating a photolithographic system currently used in the process for manufacturing semiconductor devices.
Reference Numeral 1 designates a Hg lamp used as a light source. Light passing through the photolithographic system is rendered parallel by an oblong mirror 2, a lens system 3, and a fly's-eye lens 4. Then a circular aperture 5 limits to a definite area the region transmitting light (such region is circular as shown by 5b in the front view of FIG. 12a). Light reflected at a reflection mirror 6 passes through an illumination lens 7 and then illuminates a photolithographic mask 8 (these systems are generally referred to as illumination optical system 1a). Patterns formed on the photolithographic mask 8 are demagnified to one fifth of the original by a projection lens 9 to be transcribed on a semiconductor substrate 10.
Such projection photolithographic system forms an image by allowing the 0th-order diffracted light and the +1st- or the -1st-order diffracted light to strike the projection lens. It is well known that such process establishes the following equation (1) between the dimension (D) of the mask pattern that can be resoluted, the numerical aperture (NA) of the projection lens and the wavelength of exposure light; EQU D=k.multidot..lambda./NA (1)
where k is a constant called a process constant.
Thus conventional methods for transcribing fine patterns include such measures as either shortening the wavelength of photolithographic light or enlarging the NA of the projection lens. Out of the two measures, enlarging the NA of the projection lens is not considered to be an appropriate method since the depth of focus (DOF) reduces at a rate inversely proportional to the NA squared as shown in equation 2. EQU DOF=f.multidot..lambda./NA.sup.2 ( 2)
where f is a constant called a process constant.
Thus, photolithographic systems usually used in manufacturing semiconductor device finds it very difficult to make a remarkable improvement in resolution without reducing the DOF. Consequently a demand has been made for a simple method for improving the resolution without deteriorating the DOF.
As a method for improving the resolution and the DOF in the above projection photolithographic system, a phase shift mask is proposed in recent years. However, the phase shift mask has a drawback of complicated process for preparing a mask and a complicated mask design despite a large improvement made in the resolution and the DOF.
Further proposed as a method for the same purpose is the annular illumination process in which the central part of the illumination filter (.sigma. aperture) is screened from light to allow the light to strike slantwise a mask (see "Photolithography System Using Annular Illumination" carried on Proc. of 1991 Intern. MicroProcess Conference published in July, 1991). This method permit the use of the conventional mask without making any improvement or modification and is still capable of improving both the resolution and the DOF. However, the annular illumination process screens a relatively large area of the central part of the filter (.sigma. aperture) completely from light, thereby generating a large energy loss of the photolithographic light (lowered through-put).
Still further proposed as a method for improving the resolution and the DOF is the four point incident illumination process in which the illumination filter (.sigma. aperture) is changed or an illumination system itself is modified (see Japanese Laid-Open Patent No. HEI 4 (1992)-101148). This four point incident illumination process has an advantage that the conventional mask can be used without making any improvement or modification like the annular illumination process and that a larger improvement can be made in resolution and the DOF than the annular illumination process. However, the process depends largely on the mask pattern simply because the process uses the four point illumination. In extreme cases, the resolution and the DOF are deteriorated compared with ordinary illumination systems. Besides, the four point illumination process has a drawback of a large energy loss of the photolithographic light (lowered through-put).
As mentioned above, photolithographic systems normally used can hardly make a conspicuous improvement in the resolution without deteriorating the DOF. Conventionally processes such as a phase shift mask, the annular illumination process and four points illumination process have been proposed as a method for improving the resolution and the DOF but such processes have some defects. Thus demand has been made for a simple method for improving the resolution and the depth of width free from large energy loss of exposure light and dependency on patterns.